2 "Title": "Go 1.21 Release Notes",
7 NOTE: In this document and others in this directory, the convention is to
8 set fixed-width phrases with non-fixed-width spaces, as in
9 <code>hello</code> <code>world</code>.
10 Do not send CLs removing the interior tags from such phrases.
14 main ul li { margin: 0.5em 0; }
17 <h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.21</h2>
21 Go 1.21 is not yet released. These are work-in-progress
22 release notes. Go 1.21 is expected to be released in August 2023.
27 The latest Go release, version 1.21, arrives six months after <a href="/doc/go1.20">Go 1.20</a>.
28 Most of its changes are in the implementation of the toolchain, runtime, and libraries.
29 As always, the release maintains the Go 1 <a href="/doc/go1compat">promise of compatibility</a>;
30 in fact, Go 1.21 <a href="#godebug">improves upon that promise</a>.
31 We expect almost all Go programs to continue to compile and run as before.
34 <p><!-- https://go.dev/issue/57631 -->
35 Go 1.21 introduces a small change to the numbering of releases.
36 In the past, we used Go 1.<i>N</i> to refer to both the overall Go language version and release family
37 as well as the first release in that family.
38 Starting in Go 1.21, the first release is now Go 1.<i>N</i>.0.
39 Today we are releasing both the Go 1.21 language and its initial implementation, the Go 1.21.0 release.
40 These notes refer to “Go 1.21”; tools like <code>go</code> <code>version</code> will report “<code>go1.21.0</code>”
41 (until you upgrade to Go 1.21.1).
42 See “<a href="/doc/toolchain#versions">Go versions</a>” in the “Go Toolchains” documentation for details
43 about the new version numbering.
46 <h2 id="language">Changes to the language</h2>
49 Go 1.21 adds three new built-ins to the language.
52 <li><!-- https://go.dev/issue/59488 -->
53 The new functions <code>min</code> and <code>max</code> compute the
54 smallest (or largest, for <code>max</code>) value of a fixed number
56 See the language spec for
57 <a href="https://tip.golang.org/ref/spec#Min_and_max">details</a>.
59 <li><!-- https://go.dev/issue/56351 -->
60 The new function <code>clear</code> deletes all elements from a
61 map or zeroes all elements of a slice.
62 See the language spec for
63 <a href="https://tip.golang.org/ref/spec#Clear">details</a>.
68 <p><!-- https://go.dev/issue/57411 -->
69 Package initialization order is now specified more precisely. The
73 Sort all packages by import path.
75 <li>Repeat until the list of packages is empty:
78 Find the first package in the list for which all imports are
82 Initialize that package and remove it from the list.
87 This may change the behavior of some programs that rely on a
88 specific initialization ordering that was not expressed by explicit
89 imports. The behavior of such programs was not well defined by the
90 spec in past releases. The new rule provides an unambiguous definition.
94 Multiple improvements that increase the power and precision of type inference have been made.
97 <li><!-- https://go.dev/issue/59338 -->
98 A (possibly partially instantiated generic) function may now be called with arguments that are
99 themselves (possibly partially instantiated) generic functions.
100 The compiler will attempt to infer the missing type arguments of the callee (as before) and,
101 for each argument that is a generic function that is not fully instantiated,
102 its missing type arguments (new).
103 Typical use cases are calls to generic functions operating on containers
104 (such as <a href="/pkg/slices#IndexFunc">slices.IndexFunc</a>) where a function argument
105 may also be generic, and where the type argument of the called function and its arguments
106 are inferred from the container type.
107 More generally, a generic function may now be used without explicit instantiation when
108 it is assigned to a variable or returned as a result value if the type arguments can
109 be inferred from the assignment.
111 <li><!-- https://go.dev/issue/60353, https://go.dev/issue/57192, https://go.dev/issue/52397, https://go.dev/issue/41176 -->
112 Type inference now also considers methods when a value is assigned to an interface:
113 type arguments for type parameters used in method signatures may be inferred from
114 the corresponding parameter types of matching methods.
116 <li><!-- https://go.dev/issue/51593 https://go.dev/issue/39661 -->
117 Similarly, since a type argument must implement all the methods of its corresponding constraint,
118 the methods of the type argument and constraint are matched which may lead to the inference of
119 additional type arguments.
121 <li><!-- https://go.dev/issue/58671 -->
122 If multiple untyped constant arguments of different kinds (such as an untyped int and
123 an untyped floating-point constant) are passed to parameters with the same (not otherwise
124 specified) type parameter type, instead of an error, now type inference determines the
125 type using the same approach as an operator with untyped constant operands.
126 This change brings the types inferred from untyped constant arguments in line with the
127 types of constant expressions.
129 <li><!-- https://go.dev/issue/59750 -->
130 Type inference is now precise when matching corresponding types in assignments:
131 component types (such as the elements of slices, or the parameter types in function signatures)
132 must be identical (given suitable type arguments) to match, otherwise inference fails.
133 This change produces more accurate error messages:
134 where in the past type inference may have succeeded incorrectly and lead to an invalid assignment,
135 the compiler now reports an inference error if two types can't possibly match.
139 <p><!-- https://go.dev/issue/58650 -->
140 More generally, the description of
141 <a href="https://tip.golang.org/ref/spec#Type_inference">type inference</a>
142 in the language spec has been clarified.
143 Together, all these changes make type inference more powerful and inference failures less surprising.
146 <!-- https://go.dev/issue/57969 -->
148 Go 1.21 includes a preview of a language change we are considering for a future version of Go:
149 making for loop variables per-iteration instead of per-loop, to avoid accidental sharing bugs.
150 For details about how to try that language change, see <a href="https://go.dev/wiki/LoopvarExperiment">the LoopvarExperiment wiki page</a>.
153 <h2 id="tools">Tools</h2>
155 Go 1.21 adds improved support for backwards compatibility and forwards compatibility
159 <p><!-- https://go.dev/issue/56986 -->
160 To improve backwards compatibility, Go 1.21 formalizes
161 Go's use of the GODEBUG environment variable to control
162 the default behavior for changes that are non-breaking according to the
163 <a href="/doc/go1compat">compatibility policy</a>
164 but nonetheless may cause existing programs to break.
165 (For example, programs that depend on buggy behavior may break
166 when a bug is fixed, but bug fixes are not considered breaking changes.)
167 When Go must make this kind of behavior change,
168 it now chooses between the old and new behavior based on the
169 <code>go</code> line in the workspace's <code>go.work</code> file
170 or else the main module's <code>go.mod</code> file.
171 Upgrading to a new Go toolchain but leaving the <code>go</code> line
172 set to its original (older) Go version preserves the behavior of the older
174 With this compatibility support, the latest Go toolchain should always
175 be the best, most secure, implementation of an older version of Go.
176 See “<a href="/doc/godebug">Go, Backwards Compatibility, and GODEBUG</a>” for details.
179 <p><!-- https://go.dev/issue/57001 -->
180 To improve forwards compatibility, Go 1.21 now reads the <code>go</code> line
181 in a <code>go.work</code> or <code>go.mod</code> file as a strict
182 minimum requirement: <code>go</code> <code>1.21.0</code> means
183 that the workspace or module cannot be used with Go 1.20 or with Go 1.21rc1.
184 This allows projects that depend on fixes made in later versions of Go
185 to ensure that they are not used with earlier versions.
186 It also gives better error reporting for projects that make use of new Go features:
187 when the problem is that a newer Go version is needed,
188 that problem is reported clearly, instead of attempting to build the code
189 and instead printing errors about unresolved imports or syntax errors.
193 To make these new stricter version requirements easier to manage,
194 the <code>go</code> command can now invoke not just the toolchain
195 bundled in its own release but also other Go toolchain versions found in the PATH
196 or downloaded on demand.
197 If a <code>go.mod</code> or <code>go.work</code> <code>go</code> line
198 declares a minimum requirement on a newer version of Go, the <code>go</code>
199 command will find and run that version automatically.
200 The new <code>toolchain</code> directive sets a suggested minimum toolchain to use,
201 which may be newer than the strict <code>go</code> minimum.
202 See “<a href="/doc/toolchain">Go Toolchains</a>” for details.
205 <h3 id="go-command">Go command</h3>
207 <p><!-- https://go.dev/issue/58099, CL 474236 -->
208 The <code>-pgo</code> build flag now defaults to <code>-pgo=auto</code>,
209 and the restriction of specifying a single main package on the command
210 line is now removed. If a file named <code>default.pgo</code> is present
211 in the main package's directory, the <code>go</code> command will use
212 it to enable profile-guided optimization for building the corresponding
217 The <code>-C</code> <code>dir</code> flag must now be the first
218 flag on the command-line when used.
221 <p><!-- https://go.dev/issue/37708, CL 463837 -->
222 The new <code>go</code> <code>test</code> option
223 <code>-fullpath</code> prints full path names in test log messages,
224 rather than just base names.
227 <p><!-- https://go.dev/issue/15513, CL 466397 -->
228 The <code>go</code> <code>test</code> <code>-c</code> flag now
229 supports writing test binaries for multiple packages, each to
230 <code>pkg.test</code> where <code>pkg</code> is the package name.
231 It is an error if more than one test package being compiled has a given package name.]
234 <p><!-- https://go.dev/issue/15513, CL 466397 -->
235 The <code>go</code> <code>test</code> <code>-o</code> flag now
236 accepts a directory argument, in which case test binaries are written to that
237 directory instead of the current directory.
240 <h3 id="cgo">Cgo</h3>
242 <p><!-- CL 490819 -->
243 In files that <code>import "C"</code>, the Go toolchain now
244 correctly reports errors for attempts to declare Go methods on C types.
247 <h2 id="runtime-changes">Runtime</h2>
249 <p><!-- https://go.dev/issue/7181 -->
250 When printing very deep stacks, the runtime now prints the first 50
251 (innermost) frames followed by the bottom 50 (outermost) frames,
252 rather than just printing the first 100 frames. This makes it easier
253 to see how deeply recursive stacks started, and is especially
254 valuable for debugging stack overflows.
257 <p><!-- https://go.dev/issue/59960 -->
258 On Linux platforms that support transparent huge pages, the Go runtime
259 now manages which parts of the heap may be backed by huge pages more
260 explicitly. This leads to better utilization of memory: small heaps
261 should see less memory used (up to 50% in pathological cases) while
262 large heaps should see fewer broken huge pages for dense parts of the
263 heap, improving CPU usage and latency by up to 1%.
266 <p><!-- https://go.dev/issue/57069, https://go.dev/issue/56966 -->
267 As a result of runtime-internal garbage collection tuning,
268 applications may see up to a 40% reduction in application tail latency
269 and a small decrease in memory use. Some applications may also observe
270 a small loss in throughput.
272 The memory use decrease should be proportional to the loss in
273 throughput, such that the previous release's throughput/memory
274 tradeoff may be recovered (with little change to latency) by
275 increasing <code>GOGC</code> and/or <code>GOMEMLIMIT</code> slightly.
278 <p><!-- https://go.dev/issue/51676 -->
279 Calls from C to Go on threads created in C require some setup to prepare for
280 Go execution. On Unix platforms, this setup is now preserved across multiple
281 calls from the same thread. This significantly reduces the overhead of
282 subsequent C to Go calls from ~1-3 microseconds per call to ~100-200
283 nanoseconds per call.
286 <h2 id="compiler">Compiler</h2>
289 Profile-guide optimization (PGO), added as a preview in Go 1.20, is now ready
290 for general use. PGO enables additional optimizations on code identified as
291 hot by profiles of production workloads. As mentioned in the
292 <a href="#go-command">Go command section</a>, PGO is enabled by default for
293 binaries that contain a <code>default.pgo</code> profile in the main
294 package directory. Performance improvements vary depending on application
295 behavior, with most programs from a representative set of Go programs seeing
296 between 2 and 7% improvement from enabling PGO. See the
297 <a href="/doc/pgo">PGO user guide</a> for detailed documentation.
300 <!-- https://go.dev/issue/59959 -->
302 PGO builds can now devirtualize some interface method calls, adding a
303 concrete call to the most common callee. This enables further optimization,
304 such as inlining the callee.
309 Go 1.21 improves build speed by up to 6%, largely thanks to building the
310 compiler itself with PGO.
313 <h2 id="assembler">Assembler</h2>
315 <!-- https://go.dev/issue/58378 -->
317 On amd64, frameless nosplit assembly functions are no longer automatically marked as <code>NOFRAME</code>.
318 Instead, the <code>NOFRAME</code> attribute must be explicitly specified if desired,
319 which is already the behavior on other architectures supporting frame pointers.
320 With this, the runtime now maintains the frame pointers for stack transitions.
325 The verifier that checks for incorrect uses of <code>R15</code> when dynamic linking on amd64 has been improved.
328 <h2 id="linker">Linker</h2>
330 <p><!-- https://go.dev/issue/57302, CL 461749, CL 457455 -->
331 On windows/amd64, the linker (with help from the compiler) now emits
332 SEH unwinding data by default, which improves the integration
333 of Go applications with Windows debuggers and other tools.
336 <!-- CL 463395, CL 461315 -->
338 In Go 1.21 the linker (with help from the compiler) is now capable of
339 deleting dead (unreferenced) global map variables, if the number of
340 entries in the variable initializer is sufficiently large, and if the
341 initializer expressions are side-effect free.
344 <h2 id="library">Core library</h2>
346 <h3 id="slog">New log/slog package</h3>
348 <p><!-- https://go.dev/issue/59060, https://go.dev/issue/59141, https://go.dev/issue/59204, https://go.dev/issue/59280,
349 https://go.dev/issue/59282, https://go.dev/issue/59339, https://go.dev/issue/59345, https://go.dev/issue/61200,
350 CL 477295, CL 484096, CL 486376, CL 486415, CL 487855, CL 508195 -->
351 The new <a href="/pkg/log/slog">log/slog</a> package provides structured logging with levels.
352 Structured logging emits key-value pairs
353 to enable fast, accurate processing of large amounts of log data.
354 The package supports integration with popular log analysis tools and services.
357 <h3 id="slogtest">New testing/slogtest package</h3>
359 <p><!-- CL 487895 -->
360 The new <a href="/pkg/testing/slogtest">testing/slogtest</a> package can help
361 to validate <a href="/pkg/log/slog#Handler">slog.Handler</a> implementations.
364 <h3 id="slices">New slices package</h3>
367 <!-- https://go.dev/issue/45955, https://go.dev/issue/54768 -->
368 <!-- https://go.dev/issue/57348, https://go.dev/issue/57433 -->
369 <!-- https://go.dev/issue/58565, https://go.dev/issue/60091 -->
370 <!-- https://go.dev/issue/60546 -->
371 <!-- CL 467417, CL 468855, CL 483175, CL 496078, CL 498175, CL 502955 -->
372 The new <a href="/pkg/slices">slices</a> package provides many common
373 operations on slices, using generic functions that work with slices
377 <h3 id="maps">New maps package</h3>
379 <p><!-- https://go.dev/issue/57436, CL 464343 -->
380 The new <a href="/pkg/maps/">maps</a> package provides several
381 common operations on maps, using generic functions that work with
382 maps of any key or element type.
385 <h3 id="cmp">New cmp package</h3>
387 <p><!-- https://go.dev/issue/59488, CL 496356 -->
388 The new <a href="/pkg/cmp/">cmp</a> package defines the type
389 constraint <a href="/pkg/cmp/#Ordered"><code>Ordered</code></a> and
390 two new generic functions
391 <a href="/pkg/cmp/#Less"><code>Less</code></a>
392 and <a href="/pkg/cmp/#Compare"><code>Compare</code></a> that are
393 useful with <a href="/ref/spec/#Comparison_operators">ordered
397 <h3 id="minor_library_changes">Minor changes to the library</h3>
400 As always, there are various minor changes and updates to the library,
401 made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
403 There are also various performance improvements, not enumerated here.
406 <dl id="archive/tar"><dt><a href="/pkg/archive/tar/">archive/tar</a></dt>
408 <p><!-- https://go.dev/issue/54451, CL 491175 -->
409 The implementation of the
410 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
411 interface returned by
412 <a href="/pkg/archive/tar/#Header.FileInfo"><code>Header.FileInfo</code></a>
413 now implements a <code>String</code> method that calls
414 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
417 </dl><!-- archive/tar -->
419 <dl id="archive/zip"><dt><a href="/pkg/archive/zip/">archive/zip</a></dt>
421 <p><!-- https://go.dev/issue/54451, CL 491175 -->
422 The implementation of the
423 <a href="/pkg/io/fs/#FileInfo"><code>io/fs.FileInfo</code></a>
424 interface returned by
425 <a href="/pkg/archive/zip/#FileHeader.FileInfo"><code>FileHeader.FileInfo</code></a>
426 now implements a <code>String</code> method that calls
427 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
430 <p><!-- https://go.dev/issue/54451, CL 491175 -->
431 The implementation of the
432 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
433 interface returned by the
434 <a href="/pkg/io/fs/#ReadDirFile.ReadDir"><code>io/fs.ReadDirFile.ReadDir</code></a>
436 <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
438 <a href="/pkg/archive/zip/#Reader.Open"><code>Reader.Open</code></a>
439 now implements a <code>String</code> method that calls
440 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
443 </dl><!-- archive/zip -->
445 <dl id="bytes"><dt><a href="/pkg/bytes/">bytes</a></dt>
447 <p><!-- https://go.dev/issue/53685, CL 474635 -->
448 The <a href="/pkg/bytes/#Buffer"><code>Buffer</code></a> type
450 <a href="/pkg/bytes/#Buffer.Available"><code>Available</code></a>
451 and <a href="/pkg/bytes/#AvailableBuffer"><code>AvailableBuffer</code></a>.
452 These may be used along with the
453 <a href="/pkg/bytes/#Buffer.Write"><code>Write</code></a>
454 method to append directly to the <code>Buffer</code>.
459 <dl id="context"><dt><a href="/pkg/context/">context</a></dt>
461 <p><!-- https://go.dev/issue/40221, CL 479918 -->
462 The new <a href="/pkg/context/#WithoutCancel"><code>WithoutCancel</code></a>
463 function returns a copy of a context that is not canceled when the original
466 <p><!-- https://go.dev/issue/56661, CL 449318 -->
467 The new <a href="/pkg/context/#WithDeadlineCause"><code>WithDeadlineCause</code></a>
468 and <a href="/pkg/context/#WithTimeoutCause"><code>WithTimeoutCause</code></a>
469 functions provide a way to set a context cancellation cause when a deadline or
470 timer expires. The cause may be retrieved with the
471 <a href="/pkg/context/#Cause"><code>Cause</code></a> function.
473 <p><!-- https://go.dev/issue/57928, CL 482695 -->
474 The new <a href="/pkg/context/#AfterFunc"><code>AfterFunc</code></a>
475 function registers a function to run after a context has been cancelled.
478 <p><!-- CL 455455 -->
479 An optimization means that the results of calling
480 <a href="/pkg/context/#Background"><code>Background</code></a>
481 and <a href="/pkg/context/#TODO"><code>TODO</code></a> and
482 converting them to a shared type can be considered equal.
483 In previous releases they were always different. Comparing
484 <a href="/pkg/context/#Context"><code>Context</code></a> values
485 for equality has never been well-defined, so this is not
486 considered to be an incompatible change.
490 <dl id="crypto/elliptic"><dt><a href="/pkg/crypto/elliptic/">crypto/elliptic</a></dt>
492 <p><!-- CL 459977 -->
493 All of the <a href="/pkg/crypto/elliptic/#Curve"><code>Curve</code></a> methods have been deprecated, along with <a href="/pkg/crypto/elliptic/#GenerateKey"><code>GenerateKey</code></a>, <a href="/pkg/crypto/elliptic/#Marshal"><code>Marshal</code></a>, and <a href="/pkg/crypto/elliptic/#Unmarshal"><code>Unmarshal</code></a>. For ECDH operations, the new <a href="/pkg/crypto/ecdh/"><code>crypto/ecdh</code></a> package should be used instead. For lower-level operations, use third-party modules such as <a href="https://pkg.go.dev/filippo.io/nistec">filippo.io/nistec</a>.
496 </dl><!-- crypto/elliptic -->
498 <dl id="crypto/rand"><dt><a href="/pkg/crypto/rand/">crypto/rand</a></dt>
500 <p><!-- CL 463123 -->
501 The <a href="/pkg/crypto/rand/"><code>crypto/rand</code></a> package now uses the <code>getrandom</code> system call on NetBSD 10.0 and later.
504 </dl><!-- crypto/rand -->
506 <dl id="crypto/rsa"><dt><a href="/pkg/crypto/rsa/">crypto/rsa</a></dt>
508 <p><!-- CL 471259, CL 492935 -->
509 The performance of private RSA operations (decryption and signing) is now better than Go 1.19 for <code>GOARCH=amd64</code> and <code>GOARCH=arm64</code>. It had regressed in Go 1.20.
512 Due to the addition of private fields to <a href="/pkg/crypto/rsa/#PrecomputedValues"><code>PrecomputedValues</code></a>, <a href="/pkg/crypto/rsa/#PrivateKey.Precompute"><code>PrivateKey.Precompute</code></a> must be called for optimal performance even if deserializing (for example from JSON) a previously-precomputed private key.
514 <p><!-- https://go.dev/issue/56921, CL 459976 -->
515 The <a href="/pkg/crypto/rsa/#GenerateMultiPrimeKey"><code>GenerateMultiPrimeKey</code></a> function and the <a href="/pkg/crypto/rsa/#PrecomputedValues.CRTValues"><code>PrecomputedValues.CRTValues</code></a> field have been deprecated. <a href="/pkg/crypto/rsa/#PrecomputedValues.CRTValues"><code>PrecomputedValues.CRTValues</code></a> will still be populated when <a href="/pkg/crypto/rsa/#PrivateKey.Precompute"><code>PrivateKey.Precompute</code></a> is called, but the values will not be used during decryption operations.
518 </dl><!-- crypto/rsa -->
520 <!-- CL 483815 reverted -->
522 <dl id="crypto/sha256"><dt><a href="/pkg/crypto/sha256/">crypto/sha256</a></dt>
524 <p><!-- https://go.dev/issue/50543, CL 408795 -->
525 SHA-224 and SHA-256 operations now use native instructions when available when <code>GOARCH=amd64</code>, providing a performance improvement on the order of 3-4x.
528 </dl><!-- crypto/sha256 -->
530 <!-- CL 481478 reverted -->
531 <!-- CL 483816 reverted -->
533 <dl id="crypto/tls"><dt><a href="/pkg/crypto/tls/">crypto/tls</a></dt>
535 <p><!-- https://go.dev/issue/60105, CL 496818, CL 496820, CL 496822, CL 496821, CL 501675 -->
536 Applications can now control the content of session tickets.
539 The new <a href="/pkg/crypto/tls/#SessionState"><code>SessionState</code></a> type
540 describes a resumable session.
543 The <a href="/pkg/crypto/tls/#SessionState.Bytes"><code>SessionState.Bytes</code></a>
544 method and <a href="/pkg/crypto/tls/#ParseSessionState"><code>ParseSessionState</code></a>
545 function serialize and deserialize a <code>SessionState</code>.
548 The <a href="/pkg/crypto/tls/#Config.WrapSession"><code>Config.WrapSession</code></a> and
549 <a href="/pkg/crypto/tls/#Config.UnwrapSession"><code>Config.UnwrapSession</code></a>
550 hooks convert a <code>SessionState</code> to and from a ticket.
553 The <a href="/pkg/crypto/tls/#Config.EncryptTicket"><code>Config.EncryptTicket</code></a>
554 and <a href="/pkg/crypto/tls/#Config.DecryptTicket"><code>Config.DecryptTicket</code></a>
555 methods provide a default implementation of <code>WrapSession</code> and
556 <code>UnwrapSession</code>.
559 The <a href="/pkg/crypto/tls/#ClientSessionState.ResumptionState"><code>ClientSessionState.ResumptionState</code></a> method and
560 <a href="/pkg/crypto/tls/#NewResumptionState"><code>NewResumptionState</code></a> function
561 may be used by a <code>ClientSessionCache</code> implementation to store and
567 <p><!-- CL 497376 -->
568 The package now supports the extended master secret extension (RFC 7627),
569 and enables it by default. Additionally, the deprecation of
570 <a href="/pkg/crypto/tls/#ConnectionState.TLSUnique"><code>ConnectionState.TLSUnique</code></a>
571 has been reverted, and it is populated when a connection which uses
572 extended master secret is resumed. Session tickets produced by
573 Go pre-1.21 are not interoperable with Go 1.21, meaning connections
574 resumed across versions will fall back to full handshakes.
577 <p><!-- https://go.dev/issue/44886, https://go.dev/issue/60107, CL 493655, CL 496995 -->
578 The new <a href="/pkg/crypto/tls/#QUICConn"><code>QUICConn</code></a> type
579 provides support for QUIC implementations. Note that this is not itself
580 a QUIC implementation.
583 <p><!-- https://go.dev/issue/46308, CL 497377 -->
584 The new <a href="/pkg/crypto/tls/#VersionName"></code>VersionName</code></a> function
585 returns the name for a TLS version number.
588 </dl><!-- crypto/tls -->
590 <dl id="crypto/x509"><dt><a href="/pkg/crypto/x509/">crypto/x509</a></dt>
592 <p><!-- https://go.dev/issue/53573, CL 468875 -->
593 <a href="/pkg/crypto/x509/#RevocationList.RevokedCertificates"><code>RevocationList.RevokedCertificates</code></a> has been deprecated and replaced with the new <a href="/pkg/crypto/x509/#RevocationList.RevokedCertificateEntries"><code>RevokedCertificateEntries</code></a> field, which is a slice of <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a>. <a href="/pkg/crypto/x509/#RevocationListEntry"><code>RevocationListEntry</code></a> contains all of the fields in <a href="/pkg/crypto/x509/pkix#RevokedCertificate"><code>pkix.RevokedCertificate</code></a>, as well as the revocation reason code.
596 </dl><!-- crypto/x509 -->
598 <dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
600 <p><!-- https://go.dev/issue/56892, CL 452617 -->
602 <a href="/pkg/debug/elf/#File.DynValue"><code>File.DynValue</code></a>
603 method may be used to retrieve the numeric values listed with a
607 <p><!-- https://go.dev/issue/56887, CL 452496 -->
608 The constant flags permitted in a <code>DT_FLAGS_1</code>
609 dynamic tag are now defined with type
610 <a href="/pkg/debug/elf/#DynFlag1"><code>DynFlag1</code></a>. These
611 tags have names starting with <code>DF_1</code>.
614 <p><!-- CL 473256 -->
615 The package now defines the constant
616 <a href="/pkg/debug/elf/#COMPRESS_ZSTD"><code>COMPRESS_ZSTD</code></a>.
619 <p><!-- https://go.dev/issue/60348, CL 496918 -->
620 The package now defines the constant
621 <a href="/pkg/debug/elf/#R_PPC64_REL24_P9NOTOC"><code>R_PPC64_REL24_P9NOTOC</code></a>.
624 </dl><!-- debug/elf -->
626 <dl id="debug/pe"><dt><a href="/pkg/debug/pe/">debug/pe</a></dt>
628 <p><!-- CL 488475 -->
629 Attempts to read from a section containing uninitialized data
631 <a href="/pkg/debug/pe/#Section.Data"><code>Section.Data</code></a>
632 or the reader returned by <a href="/pkg/debug/pe/#Section.Open"><code>Section.Open</code></a>
636 </dl><!-- debug/pe -->
638 <dl id="embed"><dt><a href="/pkg/embed/">embed</a></dt>
640 <p><!-- https://go.dev/issue/57803, CL 483235 -->
641 The <a href="/pkg/io/fs/#File"><code>io/fs.File</code></a>
643 <a href="/pkg/embed/#FS.Open"><code>FS.Open</code></a> now
644 has a <code>ReadAt</code> method that
645 implements <a href="/pkg/io/#ReaderAt"><code>io.ReaderAt</code></a>.
648 <p><!-- https://go.dev/issue/54451, CL 491175 -->
649 Calling <code><a href="/pkg/embed/FS.Open">FS.Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
650 will return a type that now implements a <code>String</code>
652 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
657 <dl id="errors"><dt><a href="/pkg/errors/">errors</a></dt>
659 <p><!-- https://go.dev/issue/41198, CL 473935 -->
661 <a href="/pkg/errors/#ErrUnsupported"><code>ErrUnsupported</code></a>
662 error provides a standardized way to indicate that a requested
663 operation may not be performed because it is unsupported.
664 For example, a call to
665 <a href="/pkg/os/#Link"><code>os.Link</code></a> when using a
666 file system that does not support hard links.
671 <dl id="flag"><dt><a href="/pkg/flag/">flag</a></dt>
673 <p><!-- https://go.dev/issue/53747, CL 476015 -->
674 The new <a href="/pkg/flag/#BoolFunc"><code>BoolFunc</code></a>
676 <a href="/pkg/flag/#FlagSet.BoolFunc"><code>FlagSet.BoolFunc</code></a>
677 method define a flag that does not require an argument and calls
678 a function when the flag is used. This is similar to
679 <a href="/pkg/flag/#Func"><code>Func</code></a> but for a
683 <p><!-- CL 480215 -->
685 (via <a href="/pkg/flag/#Bool"><code>Bool</code></a>,
686 <a href="/pkg/flag/#BoolVar"><code>BoolVar</code></a>,
687 <a href="/pkg/flag/#Int"><code>Int</code></a>,
688 <a href="/pkg/flag/#IntVar"><code>IntVar</code></a>, etc.)
689 will panic if <a href="/pkg/flag/#Set"><code>Set</code></a> has
690 already been called on a flag with the same name. This change is
691 intended to detect cases where <a href="#language">changes in
692 initialization order</a> cause flag operations to occur in a
693 different order than expected. In many cases the fix to this
694 problem is to introduce a explicit package dependence to
695 correctly order the definition before any
696 <a href="/pkg/flag/#Set"><code>Set</code></a> operations.
701 <dl id="go/ast"><dt><a href="/pkg/go/ast/">go/ast</a></dt>
703 <p><!-- https://go.dev/issue/28089, CL 487935 -->
704 The new <a href="/pkg/go/ast/#IsGenerated"><code>IsGenerated</code></a> predicate
705 reports whether a file syntax tree contains the
706 <a href="https://go.dev/s/generatedcode">special comment</a>
707 that conventionally indicates that the file was generated by a tool.
712 <p><!-- https://go.dev/issue/59033, CL 476276 -->
714 <a href="/pkg/go/ast/#File.GoVersion"><code>File.GoVersion</code></a>
715 field records the minimum Go version required by
716 any <code>//go:build</code> or <code>// +build</code>
722 <dl id="go/build"><dt><a href="/pkg/go/build/">go/build</a></dt>
724 <p><!-- https://go.dev/issue/56986, CL 453603 -->
725 The package now parses build directives (comments that start
726 with <code>//go:</code>) in file headers (before
727 the <code>package</code> declaration). These directives are
729 <a href="/pkg/go/build#Package"><code>Package</code></a> fields
730 <a href="/pkg/go/build#Package.Directives"><code>Directives</code></a>,
731 <a href="/pkg/go/build#Package.TestDirectives"><code>TestDirectives</code></a>,
733 <a href="/pkg/go/build#Package.XTestDirectives"><code>XTestDirectives</code></a>.
736 </dl><!-- go/build -->
738 <dl id="go/build/constraint"><dt><a href="/pkg/go/build/constraint/">go/build/constraint</a></dt>
740 <p><!-- https://go.dev/issue/59033, CL 476275 -->
742 <a href="/pkg/go/build/constraint/#GoVersion"><code>GoVersion</code></a>
743 function returns the minimum Go version implied by a build
747 </dl><!-- go/build/constraint -->
749 <dl id="go/token"><dt><a href="/pkg/go/token/">go/token</a></dt>
751 <p><!-- https://go.dev/issue/57708, CL 464515 -->
752 The new <a href="/pkg/go/token/#File.Lines"><code>File.Lines</code></a> method
753 returns the file's line-number table in the same form as accepted by
754 <code>File.SetLines</code>.
757 </dl><!-- go/token -->
759 <dl id="go/types"><dt><a href="/pkg/go/types/">go/types</a></dt>
761 <p><!-- https://go.dev/issue/61175, CL 507975 -->
762 The new <a href="/pkg/go/types/#Package.GoVersion"><code>Package.GoVersion</code></a>
763 method returns the Go language version used to check the package.
766 </dl><!-- go/types -->
768 <dl id="hash/maphash"><dt><a href="/pkg/hash/maphash/">hash/maphash</a></dt>
770 <p><!-- https://go.dev/issue/47342, CL 468795 -->
771 The <code>hash/maphash</code> package now has a pure Go implementation, selectable with the <code>purego</code> build tag.
774 </dl><!-- hash/maphash -->
776 <dl id="html/template"><dt><a href="/pkg/html/template/">html/template</a></dt>
778 <p><!-- https://go.dev/issue/59584, CL 496395 -->
780 <a href="/pkg/html/template/#ErrJSTemplate"><code>ErrJSTemplate</code></a>
781 is returned when an action appears in a JavaScript template
782 literal. Previously an unexported error was returned.
785 </dl><!-- html/template -->
787 <dl id="io/fs"><dt><a href="/pkg/io/fs/">io/fs</a></dt>
789 <p><!-- https://go.dev/issue/54451, CL 489555 -->
791 <a href="/pkg/io/fs/#FormatFileInfo"><code>FormatFileInfo</code></a>
792 function returns a formatted version of a
793 <a href="/pkg/io/fs/#FileInfo"><code>FileInfo</code></a>.
795 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>
796 function returns a formatted version of a
797 <a href="/pkg/io/fs/#FileInfo"><code>DirEntry</code></a>.
798 The implementation of
799 <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
801 <a href="/pkg/io/fs/#ReadDir"><code>ReadDir</code></a> now
802 implements a <code>String</code> method that calls
803 <a href="/pkg/io/fs/#FormatDirEntry"><code>FormatDirEntry</code></a>,
804 and the same is true for
805 the <a href="/pkg/io/fs/#DirEntry"><code>DirEntry</code></a>
807 <a href="/pkg/io/fs/#WalkDirFunc"><code>WalkDirFunc</code></a>.
812 <!-- https://go.dev/issue/56491 rolled back by https://go.dev/issue/60519 -->
813 <!-- CL 459435 reverted by CL 467255 -->
814 <!-- CL 467515 reverted by CL 499416 -->
816 <dl id="math/big"><dt><a href="/pkg/math/big/">math/big</a></dt>
818 <p><!-- https://go.dev/issue/56984, CL 453115, CL 500116 -->
819 The new <a href="/pkg/math/big/#Int.Float64"><code>Int.Float64</code></a>
820 method returns the nearest floating-point value to a
821 multi-precision integer, along with an indication of any
822 rounding that occurred.
825 </dl><!-- math/big -->
827 <dl id="net"><dt><a href="/pkg/net/">net</a></dt>
830 <!-- https://go.dev/issue/59166, https://go.dev/issue/56539 -->
831 <!-- CL 471136, CL 471137, CL 471140 -->
832 On Linux, the <a href="/pkg/net/">net</a> package can now use
833 Multipath TCP when the kernel supports it. It is not used by
834 default. To use Multipath TCP when available on a client, call
836 <a href="/pkg/net/#Dialer.SetMultipathTCP"><code>Dialer.SetMultipathTCP</code></a>
837 method before calling the
838 <a href="/pkg/net/#Dialer.Dial"><code>Dialer.Dial</code></a> or
839 <a href="/pkg/net/#Dialer.DialContext"><code>Dialer.DialContext</code></a>
840 methods. To use Multipath TCP when available on a server, call
842 <a href="/pkg/net/#ListenConfig.SetMultipathTCP"><code>ListenConfig.SetMultipathTCP</code></a>
843 method before calling the
844 <a href="/pkg/net/#ListenConfig.Listen"><code>ListenConfig.Listen</code></a>
845 method. Specify the network as <code>"tcp"</code> or
846 <code>"tcp4"</code> or <code>"tcp6"</code> as usual. If
847 Multipath TCP is not supported by the kernel or the remote host,
848 the connection will silently fall back to TCP. To test whether a
849 particular connection is using Multipath TCP, use the
850 <a href="/pkg/net/#TCPConn.MultipathTCP"><code>TCPConn.MultipathTCP</code></a>
854 In a future Go release we may enable Multipath TCP by default on
855 systems that support it.
860 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
862 <p><!-- CL 472636 -->
863 The new <a href="/pkg/net/http#ResponseController.EnableFullDuplex"><code>ResponseController.EnableFullDuplex</code></a>
864 method allows server handlers to concurrently read from an HTTP/1
865 request body while writing the response. Normally, the HTTP/1 server
866 automatically consumes any remaining request body before starting to
867 write the response, to avoid deadlocking clients which attempt to
868 write a complete request before reading the response. The
869 <code>EnableFullDuplex</code> method disables this behavior.
872 <p><!-- https://go.dev/issue/44855, CL 382117 -->
873 The new <a href="/pkg/net/http/#ErrSchemeMismatch"><code>ErrSchemeMismatch</code></a> error is returned by <a href="/pkg/net/http/#Client"><code>Client</code></a> and <a href="/pkg/net/http/#Transport"><code>Transport</code></a> when the server responds to an HTTPS request with an HTTP response.
876 <p><!-- CL 494122 -->
877 The <a href="/pkg/net/http/">net/http</a> package now supports
878 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
879 in that the expression
880 <code>errors.Is(http.ErrNotSupported, errors.ErrUnsupported)</code>
884 </dl><!-- net/http -->
886 <dl id="os"><dt><a href="/pkg/os/">os</a></dt>
888 <p><!-- https://go.dev/issue/32558, CL 219638 -->
889 Programs may now pass an empty <code>time.Time</code> value to
890 the <a href="/pkg/os/#Chtimes"><code>Chtimes</code></a> function
891 to leave either the access time or the modification time unchanged.
894 <p><!-- CL 480135 -->
896 <a href="/pkg/os#File.Chdir"><code>File.Chdir</code></a> method
897 now changes the current directory to the file, rather than
898 always returning an error.
901 <p><!-- CL 495079 -->
902 On Unix systems, if a non-blocking descriptor is passed
903 to <a href="/pkg/os/#NewFile"><code>NewFile</code></a>, calling
904 the <a href="/pkg/os/#File.Fd"><code>File.Fd</code></a> method
905 will now return a non-blocking descriptor. Previously the
906 descriptor was converted to blocking mode.
909 <p><!-- CL 477215 -->
911 <a href="/pkg/os/#Truncate"><code>Truncate</code></a> on a
912 non-existent file used to create an empty file. It now returns
913 an error indicating that the file does not exist.
916 <p><!-- https://go.dev/issue/56899, CL 463219 -->
918 <a href="/pkg/os/#TempDir"><code>TempDir</code></a> now uses
919 GetTempPath2W when available, instead of GetTempPathW. The
920 new behavior is a security hardening measure that prevents
921 temporary files created by processes running as SYSTEM to
922 be accessed by non-SYSTEM processes.
925 <p><!-- CL 493036 -->
926 On Windows the os package now supports working with files whose
927 names, stored as UTF-16, can't be represented as valid UTF-8.
930 <p><!-- CL 463177 -->
931 On Windows <a href="/pkg/os/#Lstat"><code>Lstat</code></a> now resolves
932 symbolic links for paths ending with a path separator, consistent with its
933 behavior on POSIX platforms.
936 <p><!-- https://go.dev/issue/54451, CL 491175 -->
937 The implementation of the
938 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
939 interface returned by the
940 <a href="/pkg/os/#ReadDir"><code>ReadDir</code></a> function and
941 the <a href="/pkg/os/#File.ReadDir"><code>File.ReadDir</code></a>
942 method now implements a <code>String</code> method that calls
943 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
946 <p><!-- https://go.dev/issue/53761, CL 416775, CL 498015-->
947 The implementation of the
948 <a href="/pkg/io/fs/#FS"><code>io/fs.FS</code></a> interface returned by
949 the <a href="/pkg/os/#DirFS"><code>DirFS</code></a> function now implements
950 the <a href="/pkg/io/fs/#ReadFileFS"><code>io/fs.ReadFileFS</code></a> and
951 the <a href="/pkg/io/fs/#ReadDirFS"><code>io/fs.ReadDirFS</code></a>
957 <dl id="path/filepath"><dt><a href="/pkg/path/filepath/">path/filepath</a></dt>
960 The implementation of the
961 <a href="/pkg/io/fs/#DirEntry"><code>io/fs.DirEntry</code></a>
962 interface passed to the function argument of
963 <a href="/pkg/path/filepath/#WalkDir"><code>WalkDir</code></a>
964 now implements a <code>String</code> method that calls
965 <a href="/pkg/io/fs/#FormatDirEntry"><code>io/fs.FormatDirEntry</code></a>.
968 </dl><!-- path/filepath -->
970 <!-- CL 459455 reverted -->
972 <dl id="reflect"><dt><a href="/pkg/reflect/">reflect</a></dt>
974 <p><!-- CL 408826, CL 413474 -->
975 In Go 1.21, <a href="/pkg/reflect/#ValueOf"><code>ValueOf</code></a>
976 no longer forces its argument to be allocated on the heap, allowing
977 a <code>Value</code>'s content to be allocated on the stack. Most
978 operations on a <code>Value</code> also allow the underlying value
979 to be stack allocated.
982 <p><!-- https://go.dev/issue/55002 -->
983 The new <a href="/pkg/reflect/#Value"><code>Value</code></a>
984 method <a href="/pkg/reflect/#Value.Clear"><code>Value.Clear</code></a>
985 clears the contents of a map or zeros the contents of a slice.
986 This corresponds to the new <code>clear</code> built-in
987 <a href="#language">added to the language</a>.
990 <p><!-- https://go.dev/issue/56906, CL 452762 -->
991 The <a href="/pkg/reflect/#SliceHeader"><code>SliceHeader</code></a>
992 and <a href="/pkg/reflect/#StringHeader"><code>StringHeader</code></a>
993 types are now deprecated. In new code
994 prefer <a href="/pkg/unsafe/#Slice"><code>unsafe.Slice</code></a>,
995 <a href="/pkg/unsafe/#SliceData"><code>unsafe.SliceData</code></a>,
996 <a href="/pkg/unsafe/#String"><code>unsafe.String</code></a>,
997 or <a href="/pkg/unsafe/#StringData"><code>unsafe.StringData</code></a>.
1000 </dl><!-- reflect -->
1002 <dl id="regexp"><dt><a href="/pkg/regexp/">regexp</a></dt>
1004 <p><!-- https://go.dev/issue/46159, CL 479401 -->
1005 <a href="/pkg/regexp#Regexp"><code>Regexp</code></a> now defines
1006 <a href="/pkg/regexp#Regexp.MarshalText"><code>MarshalText</code></a>
1007 and <a href="/pkg/regexp#Regexp.UnmarshalText"><code>UnmarshalText</code></a>
1008 methods. These implement
1009 <a href="/pkg/encoding#TextMarshaler"><code>encoding.TextMarshaler</code></a>
1011 <a href="/pkg/encoding#TextUnmarshaler"><code>encoding.TextUnmarshaler</code></a>
1012 and will be used by packages such as
1013 <a href="/pkg/encoding/json">encoding/json</a>.
1016 </dl><!-- regexp -->
1018 <dl id="runtime"><dt><a href="/pkg/runtime/">runtime</a></dt>
1020 <p><!-- https://go.dev/issue/38651, CL 435337 -->
1021 Textual stack traces produced by Go programs, such as those
1022 produced when crashing, calling <code>runtime.Stack</code>, or
1023 collecting a goroutine profile with <code>debug=2</code>, now
1024 include the IDs of the goroutines that created each goroutine in
1028 <p><!-- https://go.dev/issue/57441, CL 474915 -->
1029 Crashing Go applications can now opt-in to Windows Error Reporting (WER) by setting the environment variable
1030 <code>GOTRACEBACK=wer</code> or calling <a href="/pkg/runtime/debug/#SetTraceback"><code>debug.SetTraceback("wer")</code></a>
1031 before the crash. Other than enabling WER, the runtime will behave as with <code>GOTRACEBACK=crash</code>.
1032 On non-Windows systems, <code>GOTRACEBACK=wer</code> is ignored.
1035 <p><!-- CL 447778 -->
1036 <code>GODEBUG=cgocheck=2</code>, a thorough checker of cgo pointer passing rules,
1037 is no longer available as a <a href="/pkg/runtime#hdr-Environment_Variables">debug option</a>.
1038 Instead, it is available as an experiment using <code>GOEXPERIMENT=cgocheck2</code>.
1039 In particular this means that this mode has to be selected at build time instead of startup time.
1041 <code>GODEBUG=cgocheck=1</code> is still available (and is still the default).
1044 <p><!-- https://go.dev/issue/46787, CL 367296 -->
1045 A new type <code>Pinner</code> has been added to the runtime
1046 package. <code>Pinner</code>s may be used to "pin" Go memory
1047 such that it may be used more freely by non-Go code. For instance,
1048 passing Go values that reference pinned Go memory to C code is
1049 now allowed. Previously, passing any such nested reference was
1051 <a href="https://pkg.go.dev/cmd/cgo#hdr-Passing_pointers">cgo pointer passing rules.</a>
1053 See <a href="/pkg/runtime#Pinner">the docs</a> for more details.
1056 <!-- CL 472195 no release note needed -->
1058 </dl><!-- runtime -->
1060 <dl id="runtime/metrics"><dt><a href="/pkg/runtime/metrics/">runtime/metrics</a></dt>
1062 <p><!-- https://go.dev/issue/56857, CL 497315 -->
1063 A few previously-internal GC metrics, such as live heap size, are
1066 <code>GOGC</code> and <code>GOMEMLIMIT</code> are also now
1067 available as metrics.
1070 </dl><!-- runtime/metrics -->
1072 <dl id="runtime/trace"><dt><a href="/pkg/runtime/trace/">runtime/trace</a></dt>
1074 <p><!-- https://go.dev/issue/16638 -->
1075 Collecting traces on amd64 and arm64 now incurs a substantially
1076 smaller CPU cost: up to a 10x improvement over the previous release.
1079 <p><!-- CL 494495 -->
1080 Traces now contain explicit stop-the-world events for every reason
1081 the Go runtime might stop-the-world, not just garbage collection.
1084 </dl><!-- runtime/trace -->
1086 <dl id="sync"><dt><a href="/pkg/sync/">sync</a></dt>
1088 <p><!-- https://go.dev/issue/56102, CL 451356 -->
1089 The new <a href="/pkg/sync/#OnceFunc"><code>OnceFunc</code></a>,
1090 <a href="/pkg/sync/#OnceValue"><code>OnceValue</code></a>, and
1091 <a href="/pkg/sync/#OnceValues"><code>OnceValues</code></a>
1092 functions capture a common use of <a href="/pkg/sync/#Once">Once</a> to
1093 lazily initialize a value on first use.
1098 <dl id="syscall"><dt><a href="/pkg/syscall/">syscall</a></dt>
1100 <p><!-- CL 480135 -->
1102 <a href="/pkg/syscall#Fchdir"><code>Fchdir</code></a> function
1103 now changes the current directory to its argument, rather than
1104 always returning an error.
1107 <p><!-- https://go.dev/issue/46259, CL 458335 -->
1109 <a href="/pkg/syscall#SysProcAttr"><code>SysProcAttr</code></a>
1110 has a new field <code>Jail</code> that may be used to put the
1111 newly created process in a jailed environment.
1114 <p><!-- CL 493036 -->
1115 On Windows the syscall package now supports working with files whose
1116 names, stored as UTF-16, can't be represented as valid UTF-8.
1117 The <a href="/pkg/syscall#UTF16ToString"><code>UTF16ToString</code></a>
1118 and <a href="/pkg/syscall#UTF16FromString"><code>UTF16FromString</code></a>
1119 functions now convert between UTF-16 data and
1120 <a href="https://simonsapin.github.io/wtf-8/">WTF-8</a> strings.
1121 This is backward compatible as WTF-8 is a superset of the UTF-8
1122 format that was used in earlier releases.
1125 <p><!-- CL 476578, CL 476875, CL 476916 -->
1126 Several error values match the new
1127 <a href="/pkg/errors/#ErrUnsupported"><code>errors.ErrUnsupported</code></a>,
1128 such that <code>errors.Is(err, errors.ErrUnsupported)</code>
1131 <li><code>ENOSYS</code></li>
1132 <li><code>ENOTSUP</code></li>
1133 <li><code>EOPNOTSUPP</code></li>
1134 <li><code>EPLAN9</code> (Plan 9 only)</li>
1135 <li><code>ERROR_CALL_NOT_IMPLEMENTED</code> (Windows only)</li>
1136 <li><code>ERROR_NOT_SUPPORTED</code> (Windows only)</li>
1137 <li><code>EWINDOWS</code> (Windows only)</li>
1141 </dl><!-- syscall -->
1143 <dl id="testing"><dt><a href="/pkg/testing/">testing</a></dt>
1145 <p><!-- https://go.dev/issue/37708, CL 463837 -->
1146 The new <code>-test.fullpath</code> option will print full path
1147 names in test log messages, rather than just base names.
1150 <p><!-- https://go.dev/issue/52600, CL 475496 -->
1151 The new <a href="/pkg/testing/#Testing"><code>Testing</code></a> function reports whether the program is a test created by <code>go</code> <code>test</code>.
1154 </dl><!-- testing -->
1156 <dl id="testing/fstest"><dt><a href="/pkg/testing/fstest/">testing/fstest</a></dt>
1158 <p><!-- https://go.dev/issue/54451, CL 491175 -->
1159 Calling <code><a href="/pkg/testing/fstest/MapFS.Open">Open</a>.<a href="/pkg/io/fs/#File.Stat">Stat</a></code>
1160 will return a type that now implements a <code>String</code>
1162 <a href="/pkg/io/fs/#FormatFileInfo"><code>io/fs.FormatFileInfo</code></a>.
1165 </dl><!-- testing/fstest -->
1167 <dl id="unicode"><dt><a href="/pkg/unicode/">unicode</a></dt>
1169 <p><!-- CL 456837 -->
1170 The <a href="/pkg/unicode/"><code>unicode</code></a> package and
1171 associated support throughout the system has been upgraded to
1172 <a href="https://www.unicode.org/versions/Unicode15.0.0/">Unicode 15.0.0</a>.
1174 </dl><!-- unicode -->
1176 <h2 id="ports">Ports</h2>
1178 <h3 id="darwin">Darwin</h3>
1180 <p><!-- https://go.dev/issue/57125 -->
1181 As <a href="go1.20#darwin">announced</a> in the Go 1.20 release notes,
1182 Go 1.21 requires macOS 10.15 Catalina or later;
1183 support for previous versions has been discontinued.
1186 <h3 id="windows">Windows</h3>
1188 <p><!-- https://go.dev/issue/57003, https://go.dev/issue/57004 -->
1189 As <a href="go1.20#windows">announced</a> in the Go 1.20 release notes,
1190 Go 1.21 requires at least Windows 10 or Windows Server 2016;
1191 support for previous versions has been discontinued.
1196 <!-- cmd/dist: default to GOARM=7 on all non-arm systems -->
1199 <h3 id="wasm">WebAssembly</h3>
1201 <p><!-- https://go.dev/issue/38248, https://go.dev/issue/59149, CL 489255 -->
1202 The new <code>go:wasmimport</code> directive can now be used in Go programs
1203 to import functions from the WebAssembly host.
1206 <!-- https://go.dev/issue/56100 -->
1208 The Go scheduler now interacts much more efficiently with the
1209 JavaScript event loop, especially in applications that block
1210 frequently on asynchronous events.
1214 <h3 id="wasip1">WebAssembly System Interface</h3>
1216 <p><!-- https://go.dev/issue/58141 -->
1217 Go 1.21 adds an experimental port to the <a href="https://wasi.dev/">
1218 WebAssembly System Interface (WASI)</a>, Preview 1
1219 (<code>GOOS=wasip1</code>, <code>GOARCH=wasm</code>).
1223 As a result of the addition of the new <code>GOOS</code> value
1224 "<code>wasip1</code>", Go files named <code>*_wasip1.go</code>
1225 will now be <a href="/pkg/go/build/#hdr-Build_Constraints">ignored
1226 by Go tools</a> except when that <code>GOOS</code> value is being
1228 If you have existing filenames matching that pattern, you will
1229 need to rename them.
1232 <h3 id="PPC64">ppc64/ppc64le</h3>
1234 <p><!-- go.dev/issue/44549 -->
1235 On Linux, <code>GOPPC64=power10</code> now generates PC-relative instructions, prefixed
1236 instructions, and other new Power10 instructions. On AIX, <code>GOPPC64=power10</code>
1237 generates Power10 instructions, but does not generate PC-relative instructions.
1241 When building position-independent binaries for <code>GOPPC64=power10</code>
1242 <code>GOOS=linux</code> <code>GOARCH=ppc64le</code>, users can expect reduced binary
1243 sizes in most cases, in some cases 3.5%. Position-independent binaries are built for
1244 ppc64le with the following <code>-buildmode</code> values:
1245 <code>c-archive</code>, <code>c-shared</code>, <code>shared</code>, <code>pie</code>, <code>plugin</code>.
1248 <h3 id="loong64">loong64</h3>
1250 <p><!-- go.dev/issue/53301, CL 455075, CL 425474, CL 425476, CL 425478, CL 489576 -->
1251 The <code>linux/loong64</code> port now supports <code>-buildmode=c-archive</code>,
1252 <code>-buildmode=c-shared</code> and <code>-buildmode=pie</code>.
1255 <!-- proposals for x repos that don't need to be mentioned here but
1256 are picked up by the relnote tool. -->
1257 <!-- https://go.dev/issue/54232 -->
1258 <!-- https://go.dev/issue/57051 -->
1259 <!-- https://go.dev/issue/57792 -->
1260 <!-- https://go.dev/issue/57906 -->
1261 <!-- https://go.dev/issue/58668 -->
1262 <!-- https://go.dev/issue/59016 -->
1263 <!-- https://go.dev/issue/59676 -->
1264 <!-- https://go.dev/issue/60409 -->
1265 <!-- https://go.dev/issue/61176 -->
1267 <!-- changes to cmd/api that don't need release notes. -->
1268 <!-- CL 469115, CL 469135, CL 499981 -->
1270 <!-- proposals that don't need release notes. -->
1271 <!-- https://go.dev/issue/10275 -->
1272 <!-- https://go.dev/issue/59719 -->